The present invention relates to a method and to a relative unit for overwrapping products generally and packets in particular.
The invention finds application to advantage in the tobacco industry, for end-of-line operations in which leaves or sheets cut from a strip of transparent material are used to overwrap packets containing tobacco products, and in particular packets of cigarettes, to which reference is made explicitly in the present specification albeit implying no limitation in scope.
More particularly, the present invention relates to a method for overwrapping packets substantially of rectangular prismatic shape delimited axially by two end faces, and laterally by two larger faces and two smaller or flank faces extending parallel to the longitudinal axis of the rectangular prism.
A typical overwrapping unit comprises conveyor means by which the packets are taken up together with relative sheets of material, whereupon each sheet is formed into a tubular wrap about the respective packet, coaxial with the longitudinal axis of the selfsame packet and clinging thus to the two larger faces and the two flank faces.
The two meeting edges of the sheet overlap along one flank of the packet and are sealed together so as to anchor the tubular wrap, which projects at each end beyond the end faces.
Conventionally, the overwrapping is completed by making a succession of end folds against each end face, first folding and flattening an appendage of the sheet projecting from one flank face, then the appendage projecting from the other flank face, and finally the appendages projecting from the larger faces, which are flattened one over the other.
Adopting a different overwrapping method, as disclosed for example in Europe patent no 1260442, the first appendage projecting from a first flank face of the packet can be folded against the end face before the tubular wrap is sealed.
Given the characteristics of the overwrapping materials widely employed, typically polypropylene or other heat-sealable plastics, which are highly elastic and unable to retain a fold unassisted, the conveyor means must be equipped with mechanisms capable of holding the appendages of the wrap in place after the end folds have been made.
The function of such mechanisms will cease the moment that the end folds are fastened one to another by a final heat-sealing step that completes the overwrapping operation.
The method in question nonetheless imposes definite limitations on overwrapping machines used in the industry, and while suitable for those of reciprocating design, it is not applicable to more modern machines typified by continuous motion and high output speeds.
The object of the present invention is to provide a method of overwrapping packets such as will meet the practical requirements indicated above while operating at the high output speeds of which modern cigarette makers and packers are capable.